The present invention relates to a television receiver capable of insetting in part of a television picture on the screen a picture on another channel, or a picture-in-picture television receiver (hereinafter, referred to as a PinP TV receiver).
Recently, there has been proposed a so-called small-picture inset (PinP) TV receiver in which a reduced small picture of another television program is inset on part of an original television picture for effective use of the cathode ray tube of a television receiver. Such a television receiver has been described in, for example, U.S. periodical "Electronics" Sept. 1, 1977, pp. 102-106 and German patent applications Nos. P2,413,839 and P2,628,737 filed Mar. 22, 1974 and June 25, 1976, respectively. The conception of this PinP television receiver will be described briefly with reference to FIGS. 1 to 4.
FIG. 1 is a conceptional diagram of a PinP TV receiver. In this figure, reference numeral 1 represents a television receiver, 2 a cathode ray tube (CRT), 3 a large picture, and 4 a small picture in which another program picture is reduced and then inset in part of the large picture. The program selection for the large and small pictures can be made independently.
FIGS. 2A and 2B show one example of the PinP reproduction method. Reference numeral I in FIG. 2A represents a small picture before reduction, and II in FIG. 2B a large picture with an inset small picture. Assume that the reduction rate of picture is defined as ##EQU1## when the small-picture reduction ratio is selected 1/3 for both length and width, sampling of scanning lines from the small picture I is made at the rate of one out of three lines, and the horizontal period of time is reduced to 1/3 the time base to be in synchronism with the large picture, the small picture thus processed being inset in the large picture. Scanning lines 1 to 4 show part of the scanning lines before or after the reduction.
In FIG. 3, the condition of inset small picture is indicated by the time base. At I is shown a video signal of a small picture before reduction, and at II a video signal of a large picture with an inset small picture which results from reduction of picture I. From the video signal I of small picture are sampled scanning lines at the rate of one out of three as shown in FIGS. 2A and 2B, and the sampled lines are written in a field memory III of analog memory such as bucket-brigade-devices (BBDs) or digital memory such as RAMs. The stored lines are read by use of three-fold speed clocks at small picture inset positions in the video signal II of large picture so as to compress the horizontal period of time, and thus a PinP television signal can be produced. At this time, the field memory III is required to have the capacity of storing two fields of A and B, i.e., one frame. Specifically, while the memory A is being read, the next field is written in the memory B, and while the memory B is being read, the next field is written in the memory A.
FIG. 4 is a block diagram of a conventional example of the portions associated with the present invention. Shown at 11 is an antenna, 12 a video combiner or small-picture inset circuit, 13 a video processor circuit, 14 a CRT, 21 a large-picture tuner, 22 an I-F amplifier/detector, 23 a sync. separator, 31 a small-picture tuner, 32 an I-F amplifier/detector, 33 a sync. separator, 34 and 35 first and second field memories A and B, 36 a write clock generator, and 37 a read clock generator.
A small-picture video signal produced from the I-F amplifier/detector through the tuner 31 is written in, for example, the first field memory-A 34 of which the writing operation is controlled by a signal which the write clock generator 36 generates in synchronism with a signal from the sync. separator 33. While the video signal is being written in the field memory-A 34, the video signal which has been stored one-field in advance in the field memory-B 35 is read by a clock pulse which the read clock generator 37 generates in synchronism with a sync. signal separated from a large-picture video signal by the sync. separator 23, and inset in the large-picture video signal by the small-picture inset circuit 12.
As described above, the conventional PinP TV receiver employs two field memories such as BBDs or RAMs which are alternately switched for presenting a picture in picture. In this case, the required capacity of the field memory is calculated as follows. If the scanning lines are reduced to 1/3, the number of picture elements or pixels to be sampled in each horizontal line period is 100, and the field memory is formed by a digital memory of 8-bit/pixel (or 256 gray scales), then the capacity is given by EQU 262.5.times.(1/3).times.100.times.8=70 K bits/field
where 262.5=525 lines.times.1/2 in the NTSC system. Two fields require a 140-k bit memory, which thus makes the design of the PinP TV receiver difficult to a great extent.